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United States Patent |
5,288,669
|
Grateau
,   et al.
|
February 22, 1994
|
Glasses with very high index of refraction and low density
Abstract
This invention relates to glasses exhibiting refractive indices higher than
1.880, Abbe numbers of at least 29, densities lower than 4.1, good
chemical durability and stability against devitrification, and an
integrated transmission over the range of 380-800 nm at a thickness of 10
mm greater than 79%, the glasses consisting essentially, expressed in
terms of weight percent on the oxide basis, of
______________________________________
SiO.sub.2 5-8
B.sub.2 O.sub.3 15-21
ZrO.sub.2 3-10
TiO.sub.2 7-17
Nb.sub.2 O.sub.5 20.5-26
ZrO.sub.2 + TiO.sub.2 + Nb.sub.2 O.sub.5
29-45
La.sub.2 O.sub.3 19-32
Y.sub.2 O.sub.3 0-9
CaO 8-16
SrO 0-5
BaO 0-5
MgO 0-5
CaO + SrO + BaO + MgO
8-16
ZnO 0-5
Li.sub.2 O 0-2
Na.sub.2 O 0-2
K.sub.2 O 0-2
Li.sub.2 O + Na.sub.2 O + K.sub.2 O
0-2.
______________________________________
Inventors:
|
Grateau; Luc (Paris, FR);
Laborde; Pascale (Champagne Sur Seine, FR);
Prassas; Michel (Vulaines Sur Seine, FR)
|
Assignee:
|
Corning France S.A. (Avon Cedex, FR)
|
Appl. No.:
|
017183 |
Filed:
|
February 12, 1993 |
Foreign Application Priority Data
Current U.S. Class: |
501/78; 501/79; 501/901 |
Intern'l Class: |
C03C 003/068; C03C 003/066 |
Field of Search: |
501/78,901
|
References Cited
U.S. Patent Documents
3460954 | Aug., 1969 | Young | 501/63.
|
3513004 | May., 1970 | Kohut et al. | 501/78.
|
4213786 | Jul., 1980 | Faulstich et al. | 501/78.
|
4213787 | Jul., 1980 | Faulstich et al. | 501/74.
|
4390638 | Jun., 1983 | Mennemann et al. | 501/77.
|
4439531 | Mar., 1984 | Mennemann et al. | 501/75.
|
4472511 | Sep., 1984 | Mennemann et al. | 501/78.
|
4526874 | Jul., 1985 | Marszalek et al. | 501/77.
|
4584279 | Apr., 1986 | Grabowski et al. | 501/78.
|
4612295 | Sep., 1986 | Sagara | 501/51.
|
4732876 | Mar., 1988 | Nagamine et al. | 501/78.
|
4742028 | May., 1988 | Boudot et al. | 501/78.
|
4839314 | Jun., 1989 | Boudot et al. | 501/78.
|
Foreign Patent Documents |
2010672 | Mar., 1970 | DE.
| |
1302526 | Mar., 1969 | GB.
| |
Other References
Derwent Publications Ltd., London, FB; AN 87-167506 & JP-A-62 100 449
(Ohara KK) May 9, 1987.
Derwent Publications Ltd., London, GB; AN 85-194363 & JP-A-60 122 745 (Hoya
Corp.) Jul. 1, 1985.
|
Primary Examiner: Bell; Mark L.
Assistant Examiner: Sample; David R.
Attorney, Agent or Firm: Janes, Jr.; Clinton S.
Claims
We claim:
1. Glasses characterized by the fact that they exhibit an index of
refraction higher than 1.880, an Abbe number of at least 29, a density
lower than 4.1 g/cm.sup.3, a good chemical durability and a lower tendency
towards crystallization, and an integrated transmission from 380-800
nm--at a thickness of 10 mm--higher than 79%, these glasses consisting
essentially, expressed in weight percent on the oxide basis, of
______________________________________
SiO.sub.2 5-8
B.sub.2 O.sub.3 15-21
ZrO.sub.2 3-10
TiO.sub.2 7-17
Nb.sub.2 O.sub.5 20.5-26
ZrO.sub.2 + TiO.sub.2 + Nb.sub.2 O.sub.5
29-45
La.sub.2 O.sub.3 19-32
Y.sub.2 O.sub.3 0-9
CaO 8-16
SrO 0-5
BaO 0-5
MgO 0-5
CaO + SrO + BaO + MgO
8-16
ZnO 0-5
Li.sub.2 O 0-2
Na.sub.2 O 0-2
K.sub.2 O 0-2
Li.sub.2 O + Na.sub.2 O + K.sub.2 O
0-2.
______________________________________
2. Glass according to claim 1, characterized by the fact that it consists
essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________
SiO.sub.2 6-8
B.sub.2 O.sub.3 17-18
ZrO.sub.2 6-10
TiO.sub.2 7-11
Nb.sub.2 O.sub.5 20.5-26
ZrO.sub.2 + TiO.sub.2 + Nb.sub.2 O.sub.5
34-41
La.sub.2 O.sub.3 19-26
Y.sub.2 O.sub.3 0-8
CaO 12-16
SrO 0-5
BaO 0-5
MgO 0-5
CaO + SrO + BaO + MgO
12-16
ZnO 0-5
Li.sub.2 O 0-2
Na.sub.2 O 0-2
K.sub.2 O 0-2
Li.sub.2 O + Na.sub.2 O + K.sub.2 O
0-2.
______________________________________
3. Glasses according to claim 1, characterized by the fact that they also
contain up to 0.5 wt % As.sub.2 O.sub.3.
4. Glasses according to claim 1, characterized by the fact that they
contain a maximum of 10 ppm of Fe.sub.2 O.sub.3.
Description
BACKGROUND OF THE INVENTION
The present invention relates to glass compositions for the manufacturing
of lenses for optical and/or ophthalmic use, with an index of refraction
(n.sub.d) higher than 1.88, a low dispersion (Abbe number greater than or
equal to 29), and a density lower than approximately 4.g/cm.sup.3 .
For spectacle glasses of high correction glasses, the use of a glass with
an index of refraction of approximately 1.9 permits, when compared to
glasses with a lower index of refraction (1.7, for example) and equal
power, appreciable reduction of the thickness at the edge (when it is a
matter of negative power lenses) and in the center (when it is a matter of
positive power lenses).
This constitutes an undeniable advantage for the wearer of the spectacles,
in particular in case of high corrections (>6 diopters). It is also known
that increasing the index of refraction is generally accompanied by a
deterioration of the other properties characteristic of the glass, and in
particular the density increases. For a glass with high index to be able
to retain the advantages of comfort mentioned above, it is necessary for
its density not to be too high. Another disadvantage often associated with
increasing the index is the increase of dispersion (or reduction of the
Abbe number).
By reason of the above-mentioned constraints, and yet others regarding
toxicity and cost, it is difficult to find satisfactory glass compositions
with high index of refraction.
SUMMARY OF THE INVENTION
The present inventors have nevertheless succeeded in this and have found
new glass compositions belonging to the family of the lanthanum borates,
which, contrary to the majority of the other known glasses of this family,
do not contain heavy metal oxides, such as Ta.sub.2 O.sub.5 or PbO, or
toxic metal oxides, such as ThO.sub.2 or CdO, or very expensive oxides,
such as Gd.sub.2 O.sub.3 or HfO.sub.2.
More precisely, the invention relates to glasses characterized by the fact
that they exhibit an index of refraction higher than 1.880, an Abbe number
of at least 29, a density lower than 4.1 g/cm.sup.3 a low tendency towards
crystallization, and an integrated transmission from 380 to 800 nm--at a
thickness of 10 mm--higher than 79%; these glasses having the following
basic chemical composition, expressed in weight percent on the oxide
basis:
______________________________________
SiO.sub.2 5-8
B.sub.2 O.sub.3 15-21
ZrO.sub.2 3-10
TiO.sub.2 7-17
Nb.sub.2 O.sub.5 20.5-26
ZrO.sub.2 + TiO.sub.2 + Nb.sub.2 O.sub.5
29-45
La.sub.2 O.sub.3 19-32
Y.sub.2 O.sub.3 0-9
CaO 8-16
SrO 0-5
BaO 0-5
MgO 0-5
CaO + SrO + BaO + MgO
8-16
ZnO 0-5
Li.sub.2 O 0-2
Na.sub.2 O 0-2
K.sub.2 O 0-2
Li.sub.2 O + Na.sub.2 O + K.sub.2 O
0-2.
______________________________________
The ranges of proportions mentioned above for the glass constituents are
very important for obtaining the physical and chemical properties required
for the contemplated application.
In particular, the introduction of SiO.sub.2 into the composition allows
the viscosity at the liquidus to be increased slightly and the chemical
durability to be improved. A SiO.sub.2 content higher than 5% is necessary
to guarantee a good durability of the glass. The maximum SiO.sub.2 content
must not exceed 8%, the value beyond which the glass has a tendency to
crystallize.
For a B.sub.2 O.sub.3 content less than 15%, the composition is not very
stable with regard to devitrification, and beyond 21% the high indexes
expected are impossible to attain.
Among the oxides exercising a large contribution to the index of
refraction, such as ZrO.sub.2, TiO.sub.2, and Nb.sub.2 O.sub.5, a
compromise must be found between the amounts of these three elements so as
to maintain the claimed properties.
The presence of ZrO.sub.2 allows the viscosity at the liquidus to be
increased. A ZrO.sub.2 content higher than 3% contributes to the chemical
stability of the glass and improves the durability, but the tendency
towards devitrification is accentuated for contents higher than 10%.
ZrO.sub.2 exhibits the disadvantage of increasing the density of the
glass.
TiO.sub.2 in contrast allows one to produce the best index-density
compromise to be produced, and a TiO.sub.2 content higher than 7% allows a
low density to be maintained, but, with high contents, the transmission of
the glass in the visible light range decreases; a characteristic yellow
tint more or less pronounced as a function of the TiO.sub.2 content is
observed, and the Abbe number decreases rapidly. This is the reason why
its content must not exceed 17 weight percent.
Nb.sub.2 O.sub.5 also allows one to increase the index of refraction
without excessively lowering the constringence; it therefore proves
attractive, compared to TiO.sub.2, in spite of its high price. Its content
must not, however, exceed 26% in order to avoid devitrification of the
glass.
A Nb.sub.2 O.sub.5 content of at least 20.5% must be observed in order to
obtain good durability, as well as a reduced tint and a good level of
light transmission.
By maintaining the total ZrO.sub.2, TiO.sub.2, and Nb.sub.2 O.sub.5 content
between 29 and 45 weight percent, it is possible to assure a good
compromise of transmission/durability properties.
La.sub.2 O.sub.3 figures also among the oxides exercising a large
contribution to the index of refraction in like manner to ZrO.sub.2,
TiO.sub.2, and Nb.sub.2 O.sub.5 considered above. This is the compound,
however, which offers the best index-dispersion compromise. This is why it
must be present in the glass in a proportion of at least 19 weight
percent. Its content must not exceed 32 weight percent because beyond this
the addition of La.sub.2 O.sub.3 deviates disastrously with regard to the
density and devitrification of the glass.
Y.sub.2 O.sub.3 can optionally be introduced into the glass in order to
increase the constringence and to improve the chemical stability. Because
of its very high price, however, one prefers not to use it or to use it
only in a small quantity in the glass. In any case, its weight content
must not exceed 9% under pain of having a glass which is not very stable
with regard to devitrification.
CaO, SrO, BaO, MgO, and ZnO are bivalent oxides exercising a moderate
contribution to the index.
CaO, contrary to ZnO, SrO and BaO, allows one to avoid excessively
increasing the dispersion and the density. For this reason, its content
must be kept between 8 and 16 wt %. A CaO content of at least 8% is
necessary to obtain an index of refraction higher than 1.88 with at the
same time the other desired properties, particularly the Abbe number. It
must not, however, exceed 16% under pain of decreasing the acid durability
and promoting devitrification. SrO, BaO, and MgO are optional and must not
be present in a proportion higher than 5% each. Moreover, when a
combination of CaO, SrO, BaO and/or MgO is used, it must observe the
following condition: 8.ltoreq.CaO+SrO+BaO+MgO.ltoreq.16 weight percent so
as to obtain the above-mentioned properties.
Just as for the other divalent oxides except CaO, ZnO is optional, and its
content must not exceed 5 weight percent so as not to excessively increase
the density.
The oxides of alkali metals Li.sub.2 O, Na.sub.2 O, and K.sub.2 O, with
small contribution to the density, are optional and are only tolerable in
small proportions because they lead to a rapid reduction of the index of
refraction and, further, deteriorate the durability in acid medium of the
glass and promote its devitrification. Thus, their content must be
maintained at less than 2 weight percent.
Besides the ingredients above, the presence of arsenic (As.sub.2 O.sub.3)
and nitrates can be desirable for optimizing the state of
oxidation-reduction of the glass and therefore of its tint and of its
transmission, as is well known in the glass making industry in general.
Thus, for this purpose, a certain quantity of oxides can be introduced in
the form of nitrates, and a proportion of As.sub.2 O.sub.3 less than 0.5
weight percent can be introduced as a fining agent in the compositions of
the invention.
In order to obtain glass having an excellent durability in acid medium,
that is to say a weight loss in the test DIN 12116 less than 4000
mg/dm.sup.2, the glass constituents will preferably be comprised within
the ranges specified below:
______________________________________
SiO.sub.2 6-8
B.sub.2 O.sub.3 17-18
ZrO.sub.2 6-10
TiO.sub.2 7-11
Nb.sub.2 O.sub.5 20.5-26
ZrO.sub.2 + TiO.sub.2 + Nb.sub.2 O.sub.5
34-41
La.sub.2 O.sub.3 19-26
Y.sub.2 O.sub.3 0-8
CaO 12-16
SrO 0-5
BaO 0-5
MgO 0-5
CaO + SrO + BaO + MgO
12-16
ZnO 0-5
Li.sub.2 O 0-2
Na.sub.2 O 0-2
K.sub.2 O 0-2
Li.sub.2 O + Na.sub.2 O + K.sub.2 O
0-2.
______________________________________
DESCRIPTION OF PREFERRED EMBODIMENT
The invention is illustrated by the nonlimiting examples given in Table I.
The preferred example is No. 5. All the proportions are expressed in
weight percentage.
PREPARATION OF THE GLASSES
The various oxides constituting the glass are supplied by the batch
materials specified below:
______________________________________
Oxides Batch materials
______________________________________
SiO.sub.2 Quartz
B.sub.2 O.sub.3 B(OH).sub.3
ZrO.sub.2 ZrO.sub.2
TiO.sub.2 TiO.sub.2
Nb.sub.2 O.sub.5 Nb.sub.2 O.sub.5
La.sub.2 O.sub.3 La.sub.2 O.sub.5
Y.sub.2 O.sub.3 Y.sub.2 O.sub.3
CaO CaCO.sub.3, Ca(NO.sub.3).sub.2
SrO SrCO.sub.3
BaO BaCO.sub.3, Ba(NO.sub.3).sub.2
MgO MgCO.sub.3
ZnO ZnO
Li.sub.2 O Li.sub.2 CO.sub.3
Na.sub.2 O Na.sub.2 CO.sub.3, NaNO.sub.3
K.sub.2 O K.sub.2 CO.sub.3, KNO.sub.3
______________________________________
Preferably, one will choose batch materials so as to exclude as much as
possible the presence of oxides of transition metals, in particular
Fe.sub.2 O.sub.3, whose total content analyzed is advantageously kept less
than 10 ppm, in order that the glass obtained exhibits an integrated
transmission within the limits defined above.
After weighing, the various batch materials are mixed according to current
techniques. The mixture is then put in a kiln in a platinum crucible at a
temperature of approximately 1200.degree. C. When it is completely melted,
the temperature of the melt is brought to approximately
1250.degree.-1300.degree. C. for homogenization and fining. The glass melt
is then cooled to the temperature corresponding to the viscosity suitable
for molding and casting in the form of a bar.
The total duration of the operation is on the order of 2-7 hours. After
molding, the glass is annealed at approximately 600.degree.-650.degree. C.
with a rate of cooling of 60.degree. C./hour. The properties of the glass
are then determined as described below.
MEASUREMENTS OF THE PHYSICAL AND CHEMICAL PROPERTIES OF THE GLASS
The measurements of index of refraction and Abbe number are carried out
according to the usual methods (for n.sub.d, the yellow line of Hc is
used) on annealed samples. The density is measured with the aid of a
Micrometrics helium pycnometer.
The integrated transmission of the glass from 380-800 nm is determined on a
polished sample of 10 mm thickness with the aid of a Perkin-Elmer Lambda 9
spectrophotometer.
The durability in acid medium is evaluated by the test of the standard DIN
12116. It consists of determining the weight loss of a polished sample,
immersed for 6 hours in boiling 6N hydrochloric acid. The weight loss is
expressed in mg/dm.sup.2.
__________________________________________________________________________
Example
(weight %)
1 2 3 4 5 6 7 8 9 10
__________________________________________________________________________
La.sub.2 O.sub.3
23.1 24.1 21.2 21.8 22.6 24.2 22.8 22.6
22.6
22.6
Nb.sub.2 O.sub.5
23.5 21.2 23.4 23.7 23.2 21.3 23.4 23.2
23.2
23.2
TiO.sub.2
7.4 8.6 7.5 7.5 7.5 8.6 7.5 7.5 7.5 7.5
ZrO.sub.2
8.3 8.3 9.2 8.3 9.1 8.2 9.2 9.1 9.1 9.1
B.sub.2 O.sub.3
17.2 16.7 17.5 17.5 18.0 16.3 17.7 18.0
18.0
18.0
SiO.sub.2
5.6 7.2 6.1 6.1 6.0 7.2 6.1 6.1 6.1 6.1
CaO 14.9 14.0 15.1 15.1 13.5 13.4 12.9 8.5 8.5 8.5
Li.sub.2 O -- 0.4 -- -- --
Na.sub.2 O 0.8 -- -- -- --
SrO -- -- 5.0 -- --
BaO -- -- -- 5.0 --
ZnO -- -- -- -- 5.0
n.sub.d 1.889
1.885
1.884
1.884
1.883
1.881
1.885
1.883
1.886
1.889
Abbe No.
30.5 30.6 30.5 30.6 30.6 30.4 30.1 29.9
29.6
29.6
Density gkm.sup.3
4.01
4.00
3.96
3.97
3.99
3.98
3.98
4.04
4.04
4.05
T% (10 mm)
80.8 80.4 80.4 80.4 80.7 81.0 -- -- -- --
380-800 nm
Acid Durability
3421 2904 2918 3410 2785
(mg/dm.sup.2)
__________________________________________________________________________
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